Transitioning from PVC to Sealtite -- all that is needed is a threaded coupling and a T/A on the PVC.
The entire layout is weird. In my neck of the woods, the disco is always mounted directly on the structure (residential) with a whip coming out. (Usually cheesy Greenfield.)
Of course, the disco could've just been slapped on top of the PVC stub out. The extra height -- and work -- are a dead loss.